This invention relates to a method for producing disk-recording plates. Disk-recording plates are used in a videodisk system wherein information such as sounds and/or images is recorded as changes in the shape and dimension of pits (grooves) or blocks (ridges) in an information track of a disk-recording plate, and the recorded information is sensed mechanically, electrically or optically to generate image and/or sound signals which are reproduced on a receiver.
According to the videodisk system, a press master (stamper) is made by, for example, plating on the disk-recording plate and then the plated layer is peeled from the disk-recording plate to form a press master, and many press plates are made from the press master by successive transfer as in the case of preparing audio-record disks or sonosheets. These transfered plates are used as the reproducing media (that is, videodisks). Accordingly, the videodisk system has the advantage that a number of reproducing media can be manufactured at a time at very low cost as compared with other image recording-reproducing systems such as a video tape recording (VTR) system.
Disk-recording plates and press masters are manufactured, for example, by a process shown in FIGS. 1 to 5 of the drawings. FIG. 1 shows a photosensitive material for disk-recording plates which consists, for example, of a base plate 10 of an optically polished glass plate, and a positive photoresist coated thereon in a thin layer 11 (for example, about 1200 to 1500 A). The thickness of the photoresist layer is adjusted to one-fourth of the beam wavelength for reading so that when a videodisk is made by using the resulting master, the depth of pits formed in it is sufficient for the occurrence of the scattering or interference of light. Then, signals are inscribed in the photoresist layer by laser beams modulated with video signals. Subsequent development of the photoresist layer results in a disk-recording plate as shown in FIG. 2. In FIG. 2, the reference numeral 20 represents a portion which has been exposed to the irradiation of laser beams and then developed. Subsequently, as shown in FIG. 3, a thin layer of metal 30 is formed on the entire surface by known techniques such as vacuum deposition or chemical plating. Then, as shown in FIG. 4, a metal 40 such as nickel is formed on top of the metal layer 30 by electroplating as in the conventional production of audio record disks. Finally, the plated nickel layer is peeled to form a press master shown in FIG. 5. Raised portions 50 correspond to depressed portions 20 in FIG. 2. Videodisks having pits formed corresponding to the raised portions 50 in FIG. 5 are obtained by pressing the master against a thermoplastic material such as a vinyl compound and heating and cooling it in substantially the same manner as in the production of audio disk records.
This method of producing press masters for videodisks has the defect that whether an accurate recording is made on the master material (by monitoring of the recording) cannot be confirmed until after the photoresist layer has been exposed to the irradiation of laser beams and then developed, and that the depth of the pits cannot be adjusted to a constant value because the photoresist layer is difficult to coat in a uniform thickness.
Another conventional method is shown in FIGS. 6 to 10. FIG. 6 shows a starting material used in this method which consists, for example, of a flat smooth glass plate 60 and a thin metal layer 61 (200 to 300 A) formed thereon by, for example, vacuum deposition. Then, as shown in FIG. 7, the thin metal layer 61 is selectively removed by irradiating Ar laser beams modulated by video signals. The reference numeral 70 represents portions where the metal layer 61 has been removed by the laser beams. The method of forming a pattern in this manner, unlike the method described hereinabove, can permit the confirmation (by monitoring) of recording at the time of irradiating the laser beams. Since, however, the metal layer is very thin (200 to 300 A), videodisks made from this material as a disk-recording plate have pits of small depths. Consequently, the scattering or interference of light at the pits does not sufficiently occur, and the resulting products do not function as videodisks.
In order to overcome this difficulty, a negative photo-resist layer 80 is coated on the entire surface of the pattern so formed, as shown in FIG. 8. The back of the material is then exposed uniformly to the irradiation of of ultraviolet rays, and then the photoresist layer 80 is developed to produce raised portions 90, as shown in FIG. 9 (the difference in thickness between the raised portions 90 and the metal layer 61 is 1200 to 1600 A). Thus, in the state shown in FIG. 9, a raised and depressed pattern sufficient to obtain the desired depth of pits can be made, and the products can be used as disk-recording plates, whereas in the state shown in FIG. 7, it is impossible to obtain a raised and depressed pattern which permits the formation of pits having sufficient depth for the scattering or interference of light in the resulting videodisks.
In the next step, as shown in FIG. 10, a thin metal layer 100 is formed on the entire surface of the assembly shown in FIG. 9, and another metal 101 is electroplated on it. Peeling of the metal layer gives a mother master. A metal is further plated on it and then peeled to form a press master. The press master so formed can be used for the production of videodisks in quite the same way as described hereinabove.
This method has the advantage of permitting monitoring of recording, but requires complicated process steps. It also presents difficulty in that the raised portions 90 of the pattern shown in FIG. 9 are formed only in an ideal case, and actually, the scattering of light occurs at the time of uniform exposure of the photoresist layer to ultraviolet rays, and makes the shape of the raised portions distorted at their edges (for example, even when it is desired to obtain raised portions having a rectangular cross-sectional shape, the resulting raised portions become chamfered). This leads to videodisks having a markedly reduced S/N ratio.
Still another method involves forming a metal layer having a thickness of 1200 to 1600 A (which corresponds to one-fourth of the beam wavelength for reading) on a base plate, and selectively removing the metal layer by evaporation by direct application of laser beams to obtain a disk-recording plate. According to this method, monitoring can be done simultaneously with recording, and the accuracy of recording could be increased by feeding back the results of monitoring to a laser beam irradiating device. This method, however, has the defect that the metal layer having a thickness of 1200 to 1600 A is difficult to remove by laser beams with a high degree of accuracy, and the method can only permit recording of extremely low resolution.